1. Field of the Invention
The present invention relates to a metal diaphragm valve, and more particularly, to a metal diaphragm valve adapted for use in a supply system for superhigh-purity fluid.
2. Description of the Related Art
In a supply system for superhigh-purity fluid used in manufacturing processes for LSIs, the impurity concentration must be kept below 1 ppb, i.e., at a value of the ppt level. Therefore, members constituting the supply system of this type must generally fulfill the following requirements.
A. An internal fluid, which may be harmful, must be absolutely prevented from leaking out of the supply system. Even if the internal pressure of the system is negative, moreover, external fluids, such as air, must be absolutely prevented from entering the system due to reciprocal diffusion.
B. In order to prevent production of solid particles, i.e. impurities in the internal fluid, especially those particulates attributable to abrasion at sliding contact portions, the internal passage should be absolutely free from those materials (including materials for surface treatment and the like), parts or mechanisms (e.g., sliding contact portions) which tend to produce particulates.
C. In order to effect thorough removal of the impurities by purging with high efficiency, the internal passage should not have any dead-end space which allows the fluid to stagnate.
D. In order to prevent the supplying fluid from being lowered in purity by products of reaction, the constituent members should be highly resistant to corrosion by the fluid.
E. The internal fluid should be prevented from being lowered in purity by exudation of impurities adsorbed by the material surface, especially atmospheric constituents, e.g., water, adhering to those parts touched by the atmosphere. Also, there should be no use of such materials as hydrocarbon-based or high molecular materials, which easily absorb and desorb a relatively large quantity of detergent, water, and various other substances, so that the impurity concentration can be kept below a predetermined level. In short, the amount of impurities desorbed from the material constituting the internal passage should be minimized.
F. In order to accelerate the desorption of the impurities adsorbed or absorbed as described in item E, the members should be able to be baked to permit supplying and decompression of the fluid at high temperature.
G. In order to restrict the leakage rate to about 10.sup.-11 torr.multidot.1/sec (He) (current limit of a helium gas leak detector) or below, the leakage from the valve portion should be minimized in a closed state.
Items A to F, among all these requirements, are common subjects for the whole circulation system for superhigh-purity fluid. Items A to D are requirements for the case that the impurity concentration is within the range of the ppm level, while the requirements of items E and F are special factors related to the range of the ppb level.
For the gas absorption and desorption described in item E, it is known that metal generally exhibits values several figures smaller than those of the hydrocarbon-based or high molecular materials. Practically, therefore, it is advisable to use metal with less adsorption. It is to be understood, in view of the requirements of item F, that materials resistant to high temperature are preferred.
A shut-off valve provided in the aforementioned circulation system for superhigh-purity fluid naturally must fulfill the requirements of items A to F. If the requirement of item C is considered to be a factor essential to valves for the purpose, diaphragm valves are regarded as of the best suited construction.
Item G has conventionally been a pending problem peculiar to valves. In conventional valves having a valve seat formed of synthetic resin, for example, the amount of gas permeating the resin is so large that the leakage rate is as high as about 10.sup.-8 torr.multidot.1/sec (He). In order to improve the rate to about 10.sup.-11 torr.multidot.1/sec (He), a plurality of valves must be connected in series, so that the whole valve arrangement is cumbersome, and entails high costs.
Accordingly, valves best suited for use in the supply system for superhigh-purity fluid are expected to fulfill the following requirements.
H. The valve chamber should not have any dead space, there should not be any springs, stems or other driving elements, or sliding contact portions, and the metal diaphragm and the valve seat should be directly in contact with each other.
I. In order to prevent such gas permeation as the valves with a plastic valve seat undergo, the valve seat should be a metallic one integral with a valve cage.
J. In order to minimize the amount of gas adsorption and improve the sealing performance at the contact portion between the metal diaphragm and the valve seat, all the inner surfaces of the valve in contact with the fluid should be specularly finished to the smoothness of the submicron level.
In the conventional valves, however, the rate of leakage from the valve opening portion, described in item G, ranges from about 10.sup.-5 to 10.sup.-6 torr.multidot.1/sec (He), despite the fulfillment of the requirements of items H to J. Thus, the valves with a metallic valve seat are inferior to the ones with a plastic seat in the leakage rate. If the sealing performance is not satisfactory despite the submicron-level finishing of the valve opening portion, then the sealing surface pressure at the opening portion will not be uniform with respect to the circumferential direction of the diaphragm.
Conventionally, there has been proposed means for plating that portion of the metal diaphragm which faces the valve seat with soft metal, such as silver, in order to improve the sealing performance. In general, however, the soft metal is so poor in durability that it exfoliates in a short time or produces particulates by abrasion. Also, the soft metal has a low melting point and poor corrosion resistance, so that it is very hard to apply it to valves in a circulation system which uses various kinds of corrosive gases for the manufacture of semiconductors, for example.